Regional scale reservoir characterization is traditionally conducted using predominantly seismic and well log data. Additionally, well locations are selected for coring and comprehensive laboratory analysis of the cored reservoir sections. Seismic characterization focuses on identification of lithologic boundaries (seismic reflectors) which define the reservoir units and its boundaries. The seismic data is then processed to identify distribution of relevant geometric and material properties, e.g. curvature, fault identification, elastic moduli, and density, and to infer changes in inferred reservoir properties, e.g. porosity and lithology. Results are used to identify the volume of the reserves and the economic potential of the reservoir. Similarly, well log data and core data are used to constrain the seismic interpretation and provide calibrations for evaluation of the inferred properties.
Traditional interpretations of the well log data provide means for identifying and correlating lithologic bed boundaries and tracking the top and bottom boundaries of the reservoir with higher resolution. In traditional methods, the well log data is also used for interpretation of the mineralogy, bulk density, and porosity of each of the lithologies of interest, while also providing information on pore fluid types and saturations. The results provide a higher resolution evaluation of the reservoir size (number of wells) and the reservoir potential. The results also provide input for constructing and validating the geologic depositional model.
Traditionally, rock physics models of each of the dominant lithologic units, and of the reservoir unit, are developed. However, this type of modeling requires some level of homogenization of all measured properties, typically accomplished using effective media. The modeling further requires a test of consistency between all measurements, to assess model validity. For example, rock physics models allow comparisons of wave velocity without the pore fluid effect (dry rock) and, based on this comparison, the velocity of various lithologies can be determined without the effect imposed by the possibly varying pore fluid types and the pore fluid saturations. These models also provide synthetic seismic traces, i.e. numerically computed, for comparison and model validation with respect to measured seismic traces. In traditional modeling, reservoir heterogeneity exists at the lithologic bed scale, and within this scale all properties and model parameters are homogenized. Furthermore, material properties, results from parametric models, and correlations are applied laterally following the geometry of the well defined lithologic units. In brief, these traditional methods make assumptions that appear reasonable for conventional reservoirs but are unreasonable for the conditions of unconventional reservoirs.
Limitations of this traditional methodology arise when the lithologic unit is no longer the proper scale for homogenization, and this has strong implications for sampling and characterization. Methods and procedures for recognizing variability within assumed homogeneous lithologic units have been proposed, but these methods only refine the vertical resolution of the model while maintaining the assumption of lateral continuity. As explained in the following discussion, lateral continuity is a poor assumption for fine size sediments, e.g. organic-rich mudstones and fine carbonates, subjected to strong diagenetic transformations.
The dominant drivers of post-depositional transformations in organic-rich mudstones and similar systems arise from the colloidal size of the sediments, which have high surface area and high associated surface energy that promote geochemical interactions, the mixture of mineral and organic components from multiple origins and sources (terrigenous and biogenic), and from the interaction of the latter with living organisms, e.g. microorganisms that supply biogenic minerals and bacteria that feed on the deposited organic matter, both promoting a complex chain of geochemical interactions.
The effects of diagenesis in conventional, e.g. larger size, sediments are typically limited to the development of cementation and the associated reduction in porosity. This effect is limited because geochemical reactions are surface controlled, and the available surface area is relatively small. The magnitude of the surface area in the colloidal-size sediments in organic-rich mudstones is orders of magnitude larger than that of conventional reservoirs, and the geochemical interaction between their organic and inorganic components is very high. In addition, important interactions with living organisms that supply biogenic minerals metabolize the original organic matter and trigger additional geochemical transformations that do not occur homogeneously across the basin. These living organisms have particular needs and preferences regarding food supply, environment, water depth, and temperature, and thus their presence and distribution is predominantly local and changes with time. Diagenetic transformations create fundamental changes in the texture and remineralization (composition) of the original system, and develop material properties that fundamentally affect the conditions of reservoir quality (RQ) and completion quality (CQ). Most importantly, these changes are localized in time and areal extent. For example, the precipitation of biogenic silicon supplied by the silicon rich skeletons of microorganisms, if dissolved and percolated within the clay structure at the right time and prior to compaction of the sediments, results in silica strengthening of the matrix, and a matrix texture that supports high porosity and a high degree of pore interconnectivity.
Additionally, the combination of diagenetic transformations must preserve the right type, amount, and degree of degradation of organic matter in the pore space, for subsequent thermal maturation into hydrocarbons. Because the degradation of organic matter, often promoted by microbial activity, results in changes in the chemical environment, as well as the releasing of elements that promote inorganic reactions, the geochemical cycle is a complex one. Important end products result from the coincidental convergence of the right conditions of time and the presence of the right combinations of mineral and organic components undergoing specific diagenetic transformations. Since these conditions are only satisfied locally, changes in texture and composition result in the development of local regions with high reservoir and completion qualities. It is because of these complex and heterogeneous post-depositional transformations, that properties, concepts, and data correlations cannot be propagated laterally across lithologic or sub-lithologic units in organic rich mudstone systems, even when the depositional system appears to be simple. This condition needs to be well understood before any data and knowledge derived from such data can be laterally propagated across the system. Unfortunately, there is no current methodology that addresses this problem in a consistent, quantitative, and non-subjective manner. Important questions on how to obtain representative sampling in vertically and laterally heterogeneous systems, how to represent the entire system, and how to scale from small observations to the larger system behavior are still challenging. In addition, the above challenges have profound implications in the development of models, population of properties across these models, and in predicting and forecasting. The present invention solves these limitations and satisfies these needs.